Hi,
if you have only one pair of servers, I think replication via dovecot's
dsync (or doveadm via ssh) where each server holds all emails as a
local storage would be easiest.
There is a caveat with shared folders though. And dovecot replicates
only emails. The index is not included, which means
I have a small but critical server that supports our group. As a single
server - it's obviously a single-point-of-failure for lots of things.
As I just experienced...again. It was a lot more fun building systems
from components when I was younger...
Previously 3rd-party hosted solutions did